HORMONAL REGULATION OF EXERCISE C H A P T E R 5 HORMONAL REGULATION OF EXERCISE

Learning Objectives w Learn the role of your endocrine system in maintaining homeostasis in the body during rest and during acute physical activity. w Learn the difference between steroid and nonsteroid hormones and their actions within the body. w Discover the roles of hormones in regulating fat and carbohydrate metabolism. w Find out how hormones help keep your fluid levels in check and prevent dehydration.

Endocrine and Exocrine Functions w Endocrine system composed of endocrine glands—ductless glands that secrete hormones directly into the blood w Exocrine glands secrete their products into ducts (e.g. sweat glands) w Pancreas has both functions: exocrine—digestive enzymes; endocrine—insulin and glucagon

ENDOCRINE ORGANS

Hormones w Chemical messengers from endocrine glands that travel in the blood placing them in direct contact with all cells w Nonendocrine tissue can release hormones (e.g., nerve endings) w Hormones travel in the blood to their specific target organs w Receptors are specific to hormones such that only the correct hormone will “fit” the correct receptor—each cell has 2,000 to 10,000 specific receptors

Steroid Hormones w Lipid soluble w Diffuse easily through cell membranes; receptors located within cell w Chemical structure is derived from or is similar to cholesterol w Secreted by adrenal cortex (e.g., cortisol), ovaries (e.g., estrogen), testes (e.g., testosterone), placenta (e.g., estrogen)

Nonsteroid Hormones w Nonlipid soluble w Cannot easily diffuse through cell membranes; receptors located on cell membrane w Two types: amino acid derivatives (e.g., epinephrine) and protein or peptide hormones (e.g., insulin)

ACTION OF A STEROID HORMONE

ACTION OF A NONSTEROID HORMONE

Control of Hormone Release w Plasma levels of specific hormones fluctuate. w Secretion is regulated by a negative feedback system. w Cells can also alter their number of hormone receptors via down- or up-regulation.

Alteration in Number of Receptors Down-regulation—Decrease in number of cell receptors; less hormone can bind to the cell and higher concentrations of the hormone remain in the blood plasma Up-regulation—Increase in number of cell receptors; more hormone can bind to the cell and lower concentrations of the hormone remain in the blood plasma

Key Points Nature of Hormones (continued) w Hormones are classified into steroidal types (lipid soluble and formed from cholesterol) or nonsteroidal types (nonlipid soluble and formed from amino acids, peptides, or proteins). w Hormones are secreted in the blood and travel to sites where they exert an effect on only those target cells that have receptors specific to that hormone. (continued) w Steroid hormones pass through cell membranes and bind to receptors within the cell. They synthesize protein via a process called direct gene activation.

Key Points Nature of Hormones w Nonsteroid hormones bind to receptors on the cell membrane, which triggers a second messenger within the cell, which in turn triggers numerous cellular processes. w A negative feedback system regulates the release of most hormones. w The number of receptors on a cell can change the cell's sensitivity to hormones. Up-regulation is the increase of receptors and down-regulation is the decrease in receptors.

HORMONES OF THE PITUITARY GLAND

HOW ADH CONSERVES BODY WATER

Growth Hormone w Promotes muscle growth and hypertrophy by facilitating amino acid transport w Directly stimulates fat metabolism (lipolysis) w Levels are elevated during aerobic exercise in proportion to exercise intensity

Hormones of the Thyroid Gland Triiodothyronine (T3) and Thyroxine (T4) w Increase protein and enzyme synthesis w Increase size and number of mitochondria in cells w Promote rapid cellular uptake of glucose w Enhance glycolysis and glycogenesis w Increase FFA availability for oxidation Calcitonin w Decreases plasma calcium concentration w Acts primarily on bones and kidneys

Did You Know…? The parathyroid gland produces parathyroid hormone (PTH), which regulates plasma calcium and plasma phosphate concentrations by targeting the bones, intestines, and kidneys.

Hormones of the Adrenal Medulla w Catecholamines—epinephrine and norepinephrine w Stimulated by sympathetic nervous system to prepare you for immediate action w Increase rate and force of heart contraction, blood pressure, and respiration w Increase metabolic rate, glycogenolysis, and release of glucose and FFA into blood w Allow more blood to go to the skeletal muscles through vasodilation and vasoconstriction of specific vessels

Hormones of the Adrenal Cortex Mineralocorticoids w Maintain electrolyte balance in extracellular fluids w Include aldosterone Glucocorticoids w Maintain consistent plasma glucose levels between meals w Include cortisol Gonadocorticoids w Released in addition to those released by reproductive organs but in lesser amounts w Include androgens, estrogens, and progesterones

BLOOD CONCENTRATION CHANGES OF EPINEPHRINE AND NOREPINEPHRINE

Hormones of the Pancreas Insulin—secreted when plasma glucose levels are elevated (hyperglycemia) Glucagon—secreted when plasma glucose concentrations are below normal (hypoglycemia)

PLASMA GLUCOSE AND TRAINING

PLASMA INSULIN AND TRAINING

PLASMA GLUCAGON AND TRAINING

Reproductive Hormones w Androgens (i.e., testosterone) w Estrogens w Progesterone

Did You Know…? Though not a major endocrine organ, the kidneys produce a hormone called erythropoietin, which regulates red blood cell (erythrocyte) production by stimulating bone marrow cells. This hormone is important in our adaptation to training and to altitude due to the oxygen-carrying capacity of red blood cells. The kidneys also release renin, a hormone and enzyme involved in blood pressure control and fluid and electrolyte balance.

Hormones Increasing Glucose Metabolism w Glucagon w Epinephrine w Norepinephrine w Cortisol

BLOOD GLUCOSE LEVELS AFTER SPRINTING

PLASMA LEVELS OF HORMONES DURING CYCLING AT 65% VO2MAX .

PLASMA LEVELS OF GLUCOSE AND INSULIN DURING CYCLING AT 65% TO 75% VO2MAX .

Did You Know…? When carbohydrate reserves are low, the hormones accelerate the oxidation of fats to ensure your muscles get the energy they need. The rate of fat breakdown into FFA and glycerol may partly determine the rate at which muscles use fat as a fuel source during exercise.

Hormones Increasing Fat Metabolism w Cortisol w Epinephrine w Norepinephrine w Growth hormone

PLASMA LEVELS OF FFA AND CORTISOL DURING CYCLING AT 65% TO 75% VO2MAX .

PLASMA LEVELS OF EPINEPHRINE, NOREPINEPHRINE, GH, AND FFA DURING CYCLING AT 65% TO 75% VO2MAX .

Key Points Hormones, Metabolism, and Energy (continued) w Glucagon, epinephrine, norepinephrine, and cortisol help promote glycogenolysis and gluconeogenesis, thus increasing plasma glucose levels when needed. w Insulin facilitates glucose transport to the muscle fibers. (continued) w Glucose levels decline during prolonged exercise, indicating that exercise facilitates insulin action so that less is required during exercise than at rest.

Key Points Hormones, Metabolism, and Energy w When carbohydrate reserves are low, cortisol, epinephrine, norepinephrine, and growth hormone facilitate fat oxidation. w Cortisol accelerates lipolysis, releasing FFAs into the blood so they can be taken up by the cells and used for energy production. w Cortisol levels peak and return to near normal levels during prolonged exercise. When this happens, the catecholamines and GH take over cortisol's role in releasing FFAs into the blood.

Hormones and Fluid and Electrolyte Balance Aldosterone—Released by the adrenal cortex in response to decreased blood pressure; promotes sodium reabsorption in kidneys and increases plasma volume. Anitdiuretic hormone (ADH)—Released by the posterior pituitary in response to increased blood osmolarity; promotes water conservation by increasing plasma volume.

RENIN-ANGIOTENSIN MECHANISM

PLASMA VOLUME AND ALDOSTERONE DURING CYCLING

Did You Know…? Following the initial drop, plasma volume remains relatively constant throughout exercise due to 1. The actions of aldosterone and ADH, 2. Water returning from the exercising muscles to the blood, and 3. The increase in amount of water produced by metabolic oxidation within muscles.

Postexercise Fluid Balance Fluid loss from the blood results in hemoconcentration—a concentration of the particles of the blood. Hemodilution, on the other hand, is a dilution of the constituents of the blood caused by gains in fluid to the blood.

PLASMA VOLUME CHANGES

Key Points Hormones and Fluid Balance (continued) w Aldosterone and ADH are the two primary hormones involved in regulating fluid balance. w When plasma volume or blood pressure decrease, the kidneys produce renin that eventually converts to angiotensin II. (continued) w Angiotensin II increases peripheral arterial resistance, which increases blood pressure and triggers the release of aldosterone.

Key Points Hormones and Fluid Balance w Aldosterone promotes sodium reabsorption in the kidneys, which in turn causes water retention, thus increasing the plasma volume. w ADH is released in response to increased plasma osmolarity and acts on the kidneys to promote water conservation. w Plasma volume increases, which results in dilution of the plasma solutes and blood osmolarity decreases.